THIS invention relates to a casting method and apparatus and in particular to a casting method and apparatus employing an induction furnace.
An induction furnace is one in which electrically conductive material is caused to melt by induction of electrical current in it through application of a magnetic field. The material is heated as a result of power dissipation in it, attrributable to the electrical resistivity of the material as current flows through it, i.e. so-called Ohmic heating. A conventional induction furnace consists of an electrical generator or inverter which generates an alternating current or alternating voltage.
The present invention is applicable both to current-fed inverter systems, in which alternating current of fixed amplitude is generated and voltage-fed inverter systems, in which alternating voltage of fixed amplitude is generated. In either case, the current or voltage is passed through a conductive coil in the form of a solenoid in order to generate a magnetic field around the coil. The lines of magnetic flux form a toroidal shape in which the central axis of the toroid is coincident with the central axis of the coil. It is conventional practice to locate the material which is to be melted on this axis at the centre of the coil, where the magnetic flux is most dense. It is also conventional practice to accommodate the material at this position in a crucible. As melting proceeds, the crucible will contain both solid and liquid phases of the material. Even after complete melting, it may be necessary, to achieve a high quality casting, to maintain the inductive heating in order to hold the material in a super-heated state. Once the appropriate temperature has been achieved the material must be transferred to a mould or flask. This is normally achieved either by tilting the crucible in order to pour its contents through a spout or other outlet into the mould, or by accelerating the crucible in order to eject its contents under inertial effects.
In either system, the crucible is moved. This movement is problematical in that it requires, in conventional practice, disconnection of the solenoid or removal of the crucible from the solenoid. In the absence of induction heating rapid cooling of the material can take place when the crucible is moved. Such cooling may detract from the quality of the eventual casting.
It is an objective of the present invention to address this problem.